Kotsay, who is slated to bat seventh and play first in Friday's Game 1, will be the primary guy over at the position normally played by Youkilis.
The fact that the Red Sox have a veteran first baseman in Sean Casey sitting on the bench demonstrates how much confidence they have in Kotsay, despite the limited amount of time he's played the position.
"If he had played more first base in his career, you would hear people talking about how good a first baseman he is," said Red Sox manager Terry Francona. "I think the hardest thing for him is just [that] he hasn't been out there. But his footwork is good, so is his glove, [and] he's got great actions. I really think he's a very good defensive player even though he hasn't played a lot [of first], and [against] the teams that we're playing, I think defense is at a premium."
Kotsay certainly seemed up to the task when he made a tumbling, diving catch of a foul ball in the clinching Game 4 against the Los Angeles Angels of Anaheim on Monday.
"The other night, there was a high chopper [and] the runner was in the way," said Kotsay. "I didn't want to be too overly aggressive and throw it into left field, so I just took the out at first. I learned that from a former first baseman I played with named Scott Hatteberg, who rarely threw it to second base on ground balls hit to him when there was a runner on first and second. I think I've been around long enough and watched enough good first basemen to make the smart play."
The ones who are looking smart are the Red Sox, and that's for picking Kotsay up in a waiver trade with the Atlanta Braves on Aug. 27. At the time, Kotsay was brought over to help out in right field, where J.D. Drew was battling with back problems.
Drew finally recovered and is again a mainstay in the lineup.
But Lowell probably won't play again this season, so being able to plug Kotsay in at first has turned into an advantageous thing for the Red Sox, even if it wasn't something they drew up when they made the trade to get him.
"Being 110 feet away with guys like Cliff Floyd swinging the bat from the left side, I'd prefer being 290 feet away. There's a little bit more of a reaction time. I'm thrilled to just be a part of this situation and be here at the right time and be able to help out."
-- Mark Kotsay
"I knew, talking to [general manager] Theo [Epstein], the thought was that he could probably go there," Francona said. "I think Theo had maybe talked to him about it jokingly, like, 'Bring your first baseman's glove.' Not jokingly, but kind of on the lighter side.
"But right when he got here, I asked him, 'How comfortable are you over there?' He goes, 'I haven't been over there very much.' He said, 'I can do it.' I said, 'Well, start taking ground balls, and then when you're comfortable, we might put you out there, but ... I don't want to [do] it until you're ready.' I think we ended up doing it the next day. He's really done a good job."
After starting four games at first for the Red Sox in September -- his first four games there since 2006 -- Friday night's Game 1 of the ALCS marked his third start at first this postseason.
Filling in for Lowell has a little bit of meaning to it for Kotsay. They played together with the Marlins in 1999-2000.
"Mikey and I go way back," Kotsay said. "To be able to come in and help this club from the standpoint of injuries, especially with Mike's, I feel a lot of gratitude towards that."
Kotsay, who has long been lauded for his throwing arm and good instincts in the outfield, does not pretend that he'd rather be a first baseman. But there's nowhere he'd rather be than playing in October, which didn't seem likely when he was playing out the string with the Braves in late August.
"Being 110 feet away with guys like Cliff Floyd swinging the bat from the left side, I'd prefer being 290 feet away," said Kotsay. "There's a little bit more of a reaction time. I'm thrilled to just be a part of this situation and be here at the right time and be able to help out."
As for his on-the-job training at the most crucial time of year, Kotsay is trying to keep things simple.
"There are a lot of adjustments, but fielding a ground ball, it gets on you a little bit quicker than it does in the outfield, obviously, and your footwork has to be a little bit smoother," Kotsay said. "Your preparation over there, thinking through the play before it happens, will allow you to react. The reaction time is a lot quicker in what you have to do than it is in the outfield."